In 1995, aromatic hydrocarbons accounted for seven of the top thirty chemicals produced in the United States. For example, the production of benzene has grown by 5.5% over the past decade, and 16 billion pounds were produced in 1995. In addition, the total production of the other aromatic chemicals, ethylbenzene, styrene, toluene, xylenes, and cumene, exceeded 53 billion pounds in 1995. Although these chemicals are inherently found in base crude, the majority are produced in petro-chemical refineries from the conversion of saturated hydrocarbons, light naptha, by various catalytic processes. The present catalysts used in this process are based upon platinum, while an older technology was usually based upon chromia-alumina.
The elements of carbon, nitrogen, or oxygen interstitially dissolved into the lattice structure of early transition metals produce a class of materials with unique physical and chemical properties. The present interest in these materials originates from the ability of these alloys to catalyze significant industrial processes that are presently dominated by the rare and expensive Group VIII noble metals, that is, platinum, palladium, and rhodium. For example, an oxygen modified molybdenum carbide material catalytically isomerizes and reforms n-heptane. The conversion efficiencies were approximately 30% with 85% selectivity to C.sub.7 isomers. Such a process is extremely important for the production of high-octane fuels, that is, conversion of naptha to lighter, branched hydrocarbons such as methylhexanes, dimethylpentanes, and isobutane, in the refining industry. In addition, molybdenum nitrides are known to exhibit other very attractive catalytic properties such as ammonia synthesis, hydrodenitrogenation, hydrodesulfurization, and hydrogenation. For many of the processes the catalytic activity of these materials is comparable to the Group VIII metal catalysts. Tungsten carbides are known to isomerize straight chain hydrocarbons such as n-heptane to branched hydrocarbons as well.
Originally, these alloyed materials were prepared from the corresponding metal oxides by way of a high temperature, solid-state process. The resulting materials were very low surface areas, in the range of 1-5 square meters per gram (m.sup.2 /g). As a result, the catalytic efficiencies of these early materials were severely limited. Several years later, the introduction of a modified solid-state process, temperature programmed reaction, overcame this problem. In the case of nitrides, the reaction temperature of the metal oxide is slowly increased in a flow of ammonia. Materials with surface areas greater than 200 m.sup.2 /g have been prepared with this technique. As expected, many of these materials exhibit catalytic activities comparable to that of platinum based materials. However, on alumina supports, metal contents of at least 15 weight percent are usually required to achieve these catalytic activities compared to 0.5 weight percent of the Group VIII noble metals to achieve similar catalytic activities. This suggests that the majority of the metal species at the surface are not catalytically active. Other synthetic attempts were made to improve the inherent catalytic nature of these materials. These included self-propagating high temperature synthesis, and metal vapor impregnation of high surface area carbon.
A second disadvantage to the solid-state synthesis is the significant oxygen content in the materials. This is due to the greater thermodynamic stability of the initial oxide relative to both the nitride and carbide. Although in some instances this may be a advantage due to the Bronsted acid properties of the oxygen sites, the amount of oxygen in the material cannot be strictly controlled.
One object of the present invention is the development of new synthetic processes for the preparation of high surface area, Group VI alloys such as molybdenum carbonitride catalysts.
Another object of the present invention is a low temperature process, i.e., temperatures of less than about 500.degree. C., e.g., 200.degree. C. to 500.degree. C., for the preparation of high surface area, molybdenum carbonitride catalysts.
Still another object of the present invention is a resultant catalyst from the new process having a measurable increase, e.g., a 5 to 8 fold increase, in catalytic activity over catalyst materials prepared by traditional solid state approaches.